Insulators are used in many different applications. Typically, they are used to separate regions or components having different electrical potentials. In one application, an insulator (which may be referred to herein as a “source bushing”) may be used to electrically insulate an ion source from other components of an ion implanter. An ion source is a critical component of an ion implanter. The ion source generates an ion beam, which passes through the beamline of the ion implanter and is delivered to a target workpiece such as a semiconductor wafer. In order to generate this ion beam, the ion source is typically biased at a voltage significantly higher than the surrounding components, such as 80 kV. The ion source is required to generate a stable, well-defined ion beam for a variety of different ion species and extraction voltages. It is desirable to operate the ion source in a production facility for extended periods of time without the need for maintenance or repair.
To electrically insulate the ion source from the remaining components, an insulator, such as a source bushing is used to separate the components. Typically, a source bushing made using materials having an extremely higher resistivity, such as epoxy resin with ceramic filler. Such a material may have a resistance of greater than 50 GΩ (Giga Ohms) between the ion source and the grounded attached components.
An ion source typically generates an ion beam by ionizing within an arc chamber a dopant gas containing a desired dopant element. The ion beam is extracted through an extraction aperture of the ion source by an extraction electrode located proximate the extraction aperture. Different dopant gases are selected in response to the desired dopant element. Some dopant gases are fluorine containing gases such as boron trifluoride (BF3), germanium tetrafluoride (GeF4), and silicon tetrafluoride (SiF4). Because of the temperatures, voltages and ions to which it is exposed, tungsten is typically used to construct the ion chamber.
While tungsten is an excellent material to use for the ion chamber, it does tend to react with fluorine, especially after prolonged exposure, according to the equation:W+3F2→WF6 
Tungsten hexafluoride is a gaseous compound that can permeate the region in communication with the ion source. Consequently, a drawback with a conventional ion implanter utilizing fluorine containing dopant gases such as BF3, GeF4, and SiF4 is that conductive tungsten films may form on the source bushing. For example, a conductive tungsten film forms on the source bushing when the dopant gas is BF3, since fluorine creates WF6 gas in the ion source, which selectively chemically attacks the source bushing material. The tungsten film grows from one end of the source bushing and after a short time (which could be on the order of about 40 to 100 hours) could lead to electrical breakdown across the uncoated end of the source bushing.
FIG. 7 shows a simplified cross sectional view of an ion source 700, a source bushing 701 and a component 702. The component 702 may be a mounting flange or vacuum chamber, and may be made of an electrical conductive material, such as a metal or metal alloy. The component 702 is at a different electrical potential than the ion source 700, and in some embodiments, is grounded. In some embodiments, a metal shield 703 is attached to the component 702, so as to minimize electrical fields created by the ion source 700. The source bushing 701 may have one or more convolutions formed therein. These convolutions increase the surface creep path and break down voltage along the bushing (701) between the ion source (700) and component (702), on both the atmosphere and vacuum sides. In some embodiments, gases from the ion source 700, such as WF6, emerge from the ion source 700 and pass through the gap 705 between the ion source 700 and the shield 703. Tungsten hexafluoride is relatively non-reactive with metal, and therefore migrates toward the source bushing 701, which may be made using epoxy. Due to higher chemical reactivity of the epoxy, the gas chemically interacts with the bushing 701 and begins to form a film 710, starting near the ion source 700 and extending toward the component 702 (i.e. moving from left to right in FIG. 7). This film 710 has a lower resistance than the source bushing 701, and therefore affects the distribution of the voltage potential across the bushing 701.
FIG. 1 is an equivalent circuit diagram to illustrate drawbacks of a conventional source bushing having a partial coating of a tungsten film. Although this disclosure describes the film as being of tungsten, other materials can also form a conductive layer on the insulator. These materials include other metals, water or moisture. The electrical model example of FIG. 1 is based on actual measurements from a failed source bushing, which had an applied film as shown in FIG. 7. The source bushing has four convolutions, such as is shown in FIG. 7. In this embodiment and that of FIG. 5, the term “convolution” is used to describe the protrusions on the vacuum side of the source bushing. Thus, FIGS. 5 and 7 show a bushing having four convolutions. In contrast, the bushing of FIG. 3 has three convolutions, in the conventional sense. As seen in FIG. 7, the film grows so as to cover three of the four convolutions. Typically, once the film has reached point 711, arcing will begin to occur. The electrical resistance of this tungsten film is about 1.5 GΩ (Giga Ohms), which is relatively low compared to the surface resistance of the remaining electrical gap in the creepage path along the bushing surface (>20 GΩ). This creepage path is defined by the bushing surface between point of arcing 711 and the component 702, and is the portion of the bushing that remain uncoated when the arcing occurs. Therefore, the tungsten film and the remaining uncoated source bushing form a voltage divider, which applies nearly all of the extraction voltage across the uncoated part of the source bushing. For example, when the extraction voltage is 80 kilovolts (kV), nearly all (or >74 kV) of the 80 kV extraction voltage is applied across the uncoated part of the bushing. It can be seen, using these ratios, that more than 91% (or about 20 GΩ/21.5 GΩ) of the voltage is applied across the uncoated portion of the source bushing. This implies that point 711 is at 74 kV, where nearby components, such as shield 703 and component 702 are at ground. This large potential over such a small region can lead to arcing and electrical breakdown of the source bushing. As the film 710 approaches the component 702, there are multiple arc discharge paths that develop. Typically, as the film grows, it reaches a convolution point 711. Due to the shape of the bushing 701, arcing is more likely to originate at one of the points on the bushing. In one embodiment, the voltage arcs from point 711 to the tip of shield 703, as shown by dotted line 714. In another embodiment, the voltage arcs from point 711 to the extending point on the grounded component 702, such as a screw or fastener, as shown by dotted line 715. Once arcing begins, the ion implanter must be taken offline, so that the source bushing 702 can be replaced.
Accordingly, there is a need for an improved source bushing configuration which overcomes the above-described inadequacies and shortcomings. A source bushing, designed with the knowledge that a conductive film may develop on it, would be beneficial in these applications so as to prevent arcing and electrical discharge. Furthermore, it would improve the operational time (i.e. uptime) of the ion implanter, especially when fluorine-based gases are being used.